Recently, a semiconductor element (LSI: Large Scale Integration) represented by a DRAM and a flash memory is expanding its range of functions and enhancing its quality in association with the advancing communication equipment or the like, and a demand therefore is rapidly increasing (twice in two years) by the widespread use of a mobile phone, a portable music player or the like. In response to this situation, the demand for a silicon wafer as a material for a semiconductor element is also rapidly increasing. Therefore, in order to meet this increasing demand, a technique for enabling the efficient production of high-quality silicon wafers is sought after. In fact, in the semiconductor industry, a silicon wafer is produced generally by means of the Czochralski (CZ) method or the float-zone method. In the silicon wafers formed by these methods, lattice defects are involved at a certain rate. This lattice defect is a point defect made in the combination of an interstitial atom and an atomic vacancy, which mainly exists in the order of a missing one atom in a lattice. When this point defect forms an aggregate, the nature of a silicon wafer is adversely affected. Accordingly, an anneal wafer, an epitaxial wafer and a perfectly crystallized silicon wafer are in use for a so-called high-end device that is used for communication equipment or the like as described above.
Inconveniently, in an anneal wafer, an annealing process is applied to the surface of a substrate wafer to remove defects on the outermost surface of the wafer. Besides, in an epitaxial wafer, an epitaxial layer is formed on the wafer whose impurity concentration and thickness have been precisely controlled. That means, in producing anneal and epitaxial wafers, a secondary process has to be applied to a silicon wafer cut out from a silicon ingot. Hence, the number of production steps increases, leading to the difficulty in efficiently producing a silicon wafer. Besides, in anneal and epitaxial wafers, in addition to the difficulty described above, there exists another problem that the secondary process is hard to apply to an upper surface of a large-diameter wafer.
For these reasons, in recent years, a perfectly crystallized silicon wafer in which interstitial atoms have been removed to allow only atom vacancies to remain is considered to offer promising prospects. Even with the perfectly crystallized silicon wafer, however, in order to improve its yield ratio, an atom-vacancy- and an interstitial-atom-rich region have to be determined respectively inside a crystal ingot. Further, even in a single atom-vacancy-rich region, the distribution of the concentration of the atom vacancy needs to be evaluated in advance.
Accordingly, a quantitative evaluation of the concentration of atom vacancy based on the ultrasonic measurement is required for developing a growth technique of a CZ silicon crystal ingot with controlled point defects. A concentration of atomic vacancies existing within the perfectly crystallized silicon wafer manufactured by slicing the CZ silicon crystal ingot is evaluated by the ultrasonic measurement in advance, and hence, control of properties is possible in manufacturing a device by using the perfectly crystallized silicon wafer, and thus, it is expected to make a considerable contribution to improving the yield ratio of the device.
A atomic vacancy analyzing device using the ultrasonic measurement has been proposed in the past. According to this atomic vacancy analyzing device, an external magnetic field is applied to a silicon sample and then an ultrasonic wave is allowed to pass through the silicon sample which is being cooled, thus obtaining a curve indicating a relationship between a change in ultrasonic sound velocity or its absorption and a cooling temperature. Then, based on the amount of drop in the obtained steep drop curve, the concentration of defects in atomic vacancy is determined. In the silicon sample, a oscillator comprising, e.g., LiNbO3 adheres via an adhesive to the surface of a silicon wafer as a testing sample. By applying an alternating voltage to the oscillator, the generation and reception of an ultrasonic pulse are realized.
Unfortunately, a silicon wafer slightly expands at about 200K or lower, while the oscillator comprising LiNbO3 contracts at low temperatures. Hence, if such a method is employed that the typically employed oscillator comprising LiNbO3 is allowed to adhere to a silicon wafer, the differential thermal expansion causes the adhered portion between the surface of a silicon wafer and the oscillator to exfoliate from each other. Accordingly, it has been learnt that if a transducer utilizing the oscillator comprising LiNbO3 is employed, then the change in ultrasonic sound velocity in a silicon wafer can not be stably measured.
On the other hand, instead of the oscillator comprising LiNbO3, a quantitative evaluation device of an atomic vacancy has been proposed, which utilizes a thin film oscillator comprising zinc oxide (ZnO). In order to generate and receive an ultrasonic pulse, the silicon wafer, being a material under test, is provided with an ultrasonic generator on its one side and an ultrasonic receiver on its other side, These ultrasonic generator and ultrasonic receiver each comprise a transducer composed of the above oscillator comprising the ZnO thin film and electrodes provided on both sides of the silicon wafer, with the thin film oscillator being sandwiched between the electrodes. One electrode is provided on the silicon wafer via a chrome thin film. Here, the ZnO thin film is formed directly by a sputtering method on the electrode so that its C axis is approximately aligned in a certain direction. Here, the C axis means a rotational symmetrical axis of a crystalline structure of the ZnO thin film.
When an alternating voltage is applied to the electrode of the ultrasonic generator provided on one surface of the silicon wafer, the thin film oscillator expands, contracts and vibrates (actually a pulse wave) to send an elastic wave (in fact, a pulse wave) into the silicon wafer. This elastic wave is detected by the ultrasonic receiver provided on the opposing surface of the silicon wafer that is to be converted into an electric signal.
As described above, by employing the ZnO thin film as an oscillator, there occurs no exfoliation in oscillator, thus exerting a profound effect capable of stably measuring an atomic vacancy existing in the silicon wafer.